As a typical example of an electrolytic cell for electrolyzing an electrolyte solution such as sea water, salt water, or the like, there is a pipe-type electrolysis cell.
The pipe-type electrolysis cell has a pipe-type electrode typically consisting of an outer pipe and an inner pipe. The inner pipe is a combined bipolar tube electrode in which one portion serves as an anode and the other portion serves as a cathode. The outer pipe includes an anode portion, a cathode portion, and an insulating spacer disposed at a center portion thereof, in which the anode portion and the cathode portion are disposed to be opposite to the anode and the cathode of the inner pipe. Alternatively, both of the inner pipe and the outer pipe may be monopolar electrodes having one polarity.
In the pipe-type electrolytic cell, when DC power is applied between the anode and the cathode to cause electrolysis while an electrolyte solution flows along the surfaces of the inner pipe and the outer pipe, electrolyzed water is produced.
Electrolysis can be used for various process such as production of chlorine, sodium hydroxide, sodium hypochlorite, and the like through electrolysis of sea water or salt water, production of hydrogen and oxygen through electrolysis of water, production of various organic compounds through electrolysis of carbon dioxide, decomposition of ammonia or organic substances, production of acidic water and alkaline water, and the like.
Among these processes, chemical equations of a typical electrolysis process to produce sodium hypochlorite from sea water or salt water are shown below.2Cl−→Cl2+2e−  Anodic reaction:2H2O+2e−→2OH−+H2↑  Cathodic reaction:Cl2+2NaOH→NaOCl+NaCl+H2O  Bulk reaction:
Chlorine (Cl2) is produced at the anode side through oxidation of chlorine ions, and hydrogen gas (H2) and hydroxyl ions (OH—) are produced at the cathode side through water splitting. Hydroxyl ions (OH—) produced at the cathode side react with sodium ions (Na+) in a bulk phase to produce sodium hydroxide (NaOH), and the sodium hydroxide (NaOH) reacts with chlorine (Cl2), in a bulk phase, produced at the anode to produce sodium hypochlorite (NaOCl). Sodium hypochlorite (NaOCl) produced in this way is used to lower biological activity, or used in various applications for sterilization (disinfection) and cleaning.
Hardness materials such as Ca and Mg contained in an electrolyte solution form scale on a cathode electrode through chemical reactions described below, during electrolysis, and the accumulated scale lowers electrolysis efficiency, resulting in an increase in cell voltage, impedes the flow of a fluid, and causes physical damage attributable to short-circuiting between electrodes in extreme cases.
Scale formation reaction: HCO3−+NaOH→CO32−+H2O2O+Na+
Ca2+ or Mg2++CO32−→CaCO3 or MgCO3 
Ca2+ or Mg2++2OH−→Ca(OH)2 or Mg(OH)2 
A conventional technology of preventing accumulation of scale is disclosed in Korean Patent Application Publication No. 10-2006-0098445 (Electronic Water Treatment System And Method For Controlling The Same). According to this technology, an anode bar serving as an anode is installed inside a pipeline through which a fluid flows, a housing surrounding the anode bar serves as a cathode, and an electric current flows through the anode bar to form electromagnetic fields in a fluid passage, thereby preventing generation of scale. That is, when a fluid flows along the fluid passage in which electromagnetic fields are formed, since free electrons are sufficiently generated due to the electromagnetic fields, inorganic substances contained in the fluid become structurally stable, which prevents scale formation.
The conventional technology requires generation of uniform density of electromagnetic fields to suppress generation of scale. However, in the case in which the flow rate of fluid, flowing along the fluid passage, is not constant but fluctuates, it is difficult to maintain uniform density of electromagnetic fields. For this reason, it is difficult to effectively impede scale formation. That is, the conventional art, which prevents scale formation through an electrical method, requires an advanced technology to precisely control the intensity of current in accordance with the flow rate of fluid. Therefore, it is not easy to substantially perfectly prevent scale formation, and thus it is necessary to mechanically remove generated scale.
To solve the problem of this technology, Korean Patent Application No. 10-2012-0032399 (titled “Pipe-type Electrolysis Cell) is disclosed. The “Pipe-type Electrolysis Cell” provides an electrolytic cell in which corners of electrodes in a fluid passing zone are eliminated to prevent scale formation on the surface of a cathode during operation of the electrolytic cell. The construction of the pipe-type electrolysis cell is shown in FIGS. 1 to 6.
With reference to FIGS. 1 to 6, according to a conventional art, a pipe-type electrolysis cell 10 includes an insulating spacer 11 disposed at a middle portion thereof, an anode outer pipe 12 disposed on one side of the insulating spacer 11, and a cathode outer pipe 13 disposed on the other side of the insulating spacer 11. A cathode inner pipe (not shown) is installed inside the anode outer pipe 12, and an anode inner pipe 13′ is installed inside the cathode outer pipe 13. An insulating bushing 14, a spiral block 15, a fixing bushing 16, and an inlet/outlet connection nipple 17 are assembled with an end of the electrolysis cell 10 by a coupling member 18. Due to the use of the spiral block 15, when a fluid flows in and out of the electrolysis cell 10 through a spiral hole 15a formed in the spiral block 15, since a fluid passage has a spiral form, the fluid can flow at a constant uniform flow rate. This prevents hydrogen gas H2 and oxygen gas O2 generated during an electrolytic reaction from being locally concentrated in a specific portion, which removes an intervening factor of surface reaction attributable to the gases and enables uniform reaction. Therefore, it is possible to obtain effects of an improvement in efficiency of electrolytic reaction and an increase in life span of the electrolysis cell.
In addition, a plurality of electrolysis cells 10, each cell being the pipe-type electrolysis cell 10 having the structure described above, is connected in series with each other to form a unit module 20 as illustrated in FIG. 1. Therefore, it is possible to easily provide a module having desired capacity. Furthermore, a plurality of unit modules 20 may be connected in parallel with each other to increase the electrolysis capacity, as illustrated in FIG. 2.
The electrolysis module consisting of the pipe-type electrolysis cells 10 has a higher withstand voltage and a simpler structure than conventional cube-shaped electrolysis modules using a flat plate electrode. Furthermore, since this electrolysis module has an improved velocity profile, it is possible to minimize scale accumulation and facilitate hydrogen emissions.
However, in the case of the conventional pipe-type electrolysis cell, since only one surface of the electrode is involved in an electrolytic reaction, a large amount of material is likely to be wasted. In addition, since the pipe-type electrolysis cell requires a large installation space, it is difficult to use the pipe-type electrolysis cell in small places. In addition, since the number of parts of the pipe-type electrolysis cell is large and assembling of the parts is complicated, the manufacturing cost is increased.
In addition, in the case of the conventional pipe-type electrolysis cell, current distribution is non-uniform over the electrode. Therefore, when the conventional pipe-type electrolysis cells are arranged in multiple stages, it is difficult to obtain uniform reaction, the life span of the electrode is shortened, and excessive heat is generated.